METHOD FOR CONTROLLING A COMBUSTION PROCESS

Abstract: A method for controlling a combustion process in a combustion plant comprises sensing the content of NOx in the flue gases and affecting, on the basis thereof, the combustion by controlling a parameter to obtain the desired NOx content, wherein interference is caused in said at least one combustion process parameter, preferably the excess of O2 in the flue gases and/or the flow rate of a chemical supplied, e.g. urea, and generating a signal which gives successive values of the parameter and which is low-pass-filtered; measuring the NOx content and the O2 content of the flue gases by means of transducers so as to generate an NOx content signal and an O2 content signal; performing the NOx and O2 content measurements such that the signals obtained are referable to flue-gas measuring points located at any rate in the immediate vicinity of one another; low-pass-filtering the NOx and O2 content signals obtained and sampling these low-pass-filtered signals to obtain NOx and O2 content values successive in time; calculating zero-%-O2-compensated NOx comp content values by using the O2 content values obtained and then utilising O2 content values which are related in time to the corresponding NOx content values; high-pass-filtering the NOx comp content values and the successive parameter values; calculating, on the basis of the high-pass-filtered values, the relationship between NOx comp and the parameter; and controlling the parameter on the basis thereof.

Description: METHOD FOR CONTROLLING A COMBUSTION PROCESS Technical Field The present invention relates to a method for controlling a com¬ bustion process in a combustion plant, e.g. a combined power and heating plant, wherein the NO x content of the flue gases generated in the combustion is sensed and the combustion is affected on the basis thereof by controlling at least one parameter in order to maintain the NO x content on the desired level. Technical Background The NO x content of the flue gases emitted from a combustion plant (hereinafter referred to as "furnace") is affected by many factors. Among these may be mentioned the chemistry of the fuel, the furnace temperature, the oxygen excess in the flue gases and so forth. The oxygen excess and the furnace temperature are examples of para¬ meters that can be controlled by affecting the operational state of the furnace. It is however difficult to know how the change of the opera¬ tional state is to be performed in the practical case in order that the NO x emission should be reduced as desired. Tests have been conducted to calculate the link between the oxygen content of the flue gases and the NO x content. The oxygen excess O2 is then varied under controlled conditions and the mea¬ sured values of O2 and NO x are indicated in graphs. By using calcu¬ lated regression lines, it is thereafter possible to estimate the reduc- tion of NO x obtained by a certain reduction of O2. As will be readily appreciated, this calls for lengthy, well-controlled experiments, in which all other parameters that may affect the NO x content must be maintained constant in order to obtain good results. Object of the Invention One object of the present invention is to provide a method of the type stated in the introductory part, by means of which the relation¬ ship between a combustion process parameter and NO x can be deter¬ mined quickly and with high accuracy without having to change the operational state of the furnace and without any other combustion process parameters having any appreciable effect. Another object is to provide a method which is useful in real time and which thus confers the possibility of on-line control, i.e. that can be integrated in the control system of the furnace. Summary of the Invention According to the invention, the above-mentioned objects are achieved by a method having the features stated in the appended claims. The method thus comprises causing interference in the para¬ meter concerned and generating a signal which contains successive values of the parameter, and measuring the varying NO x and O2 con¬ tents of the flue gases by means of transducers, so as to generate an NO x content signal and an O2 content signal. If the parameter is the oxygen excess in the flue gases, the O2 content signal will also be the parameter signal. If the parameter is the flow rate of a chemical, e.g. urea, supplied to the furnace, the flow rate is sensed to provide a parameter signal. The NO x and O2 content signals obtained (and, where applic¬ able, also another parameter signal employed) are low-pass-fϊltered and sampled, suitably in connection with A-D conversion, to obtain NO x and O2 content values successive in time (and, where applicable, also another parameter signal employed). In order to attain high accuracy, the NO x content values are converted so as to obtain zero-%-O2-compensated NO x comp content values, while using pertaining O2 content values. In this case, it is essential that the values involved relate to events occurring at any rate substantially simultaneously in the process. Therefore, in order to ensure the correct relationship in time, the previously mentioned NO x and O2 content measurements are done so that the signals obtained are referable to flue-gas measuring points, which are located at any rate in the immediate vicinity of one another, the transport times between the measuring points and the transducers and the time constants of the transducers being also considered when linking together the pertaining NO x and O2 content values in conjunction with the compensation calculations. However, it should then be ensured that there is no prompt relationship between the NO x and O2 content values. In order to eliminate the presence of trends, transients etc., the NO x comp content values obtained are high-pass-filtered as well as the successive parameter values (which thus are the sampled O2 con¬ tent values, when the parameter is the oxygen excess in the flue gases, and the sampled flow rate values, when the parameter is a chemical flow rate), whereupon the relationship between NO x comp and the parameter at issue can be calculated. Preferably, this is done by utilising recursive process identification, in which the derivative for the relationship can be determined by step-response tests. Since this can be performed on-line and in real time, it becomes possible to quickly detect process changes and to control the process on the basis thereof, also automatically by integration in the control system of the furnace. It has been found that the use of recursive process identifica¬ tion does not involve the need of absolute process concurrence in time of the parameter values and NO x values, respectively, that can be linked to each other during the relationship calculation. This is of course an obvious advantage when the parameter is a chemical flow rate which is sensed at a point other than where O2 and NO x are sensed. When the parameter is the oxygen excess O2 in the flue gases, time compensation must however be carried out, if need be, in such a manner that the respective O2 content value linked to an NO x comp content value at any rate does not succeed the latter in time, i.e. it must be ensured that O2 is seen to control NO x and not the other way round. The controlled interference which is preferably used accord- ing to the invention is of the PRBS type (Pseudo Random Binary Sequence). This has been found to give higher frequency components of the values used for the relationship calculation, which means that the identification of the relationship can be carried out in a shorter time and with higher accuracy. The algorithm used for the relation- ship calculation thus needs fewer data for the calculations. The results of the calculations are received more quickly, and a sudden change in the process can be discovered and attended to with a shorter time delay. The recursive process identification uses a large number of successive, weighted values, the values as last received being given the highest weight. The time interval between the samples suitably is of the order of a second or seconds, and the algorithm-controlled model calculation of the process identification can be updated after but a few new samples, preferably after each new sample. It is under¬ stood that this confers the possibility of determining the derivative of the relationship at very short intervals, e.g. of the order of tens of seconds, so that the process can be followed almost continuously. According to an advantageous embodiment of the inventive method, it is possible, when the parameter concerned is a chemical flow rate, and considering the calculated relationship between the flow rate and NO x and the cost of the chemical as well as the cost repre¬ sented by the NO x environmental control charge, to control the che¬ mical flow rate quickly and efficiently towards a flow rate minimising the overall costs. In practice, the invention has been found to yield highly accu- rate measuring results, making it possible with very high prediction to induce parameter changes for affecting the NO x content of the flue gases, and this without interfering with the rest of the process. The invention will be described in more detail hereinafter with the aid of exemplary embodiments thereof, reference being made to the accompanying drawings. Brief Description of the Drawings Fig. 1 is a diagram synoptically showing how a combustion sys¬ tem comprising a furnace with a fluidised bed has been modified for the incorporation of an embodiment of the present invention; Fig. 2 is a diagram synoptically illustrating the signal process¬ ing in conjunction with the embodiment of Fig. 1 ; Fig. 3 is a diagram illustrating the process identification employed; and Fig. 4 is a graph illustrating the use of another embodiment of the invention for minimising costs when injecting urea with a view to reducing NO x . Description of Embodiments The combustion plant schematically illustrated in Fig. 1 , which has been modified for incorporating an embodiment of the invention, comprises a furnace 1 having a circulating fluidised bed, which is supplied with primary air 3 and secondary air 5 from a main air flow 7. The flue gases pass an electric filter 9 before leaving through a chimney 11. Before the electric filter 9, an O2 meter is arranged, giving a value of combustion O2. This is made use of in the furnace control process, as will be described in more detail below. Two flue-gas transducers 13, 15 are connected to the chimney via a common suction pipe 17. The transducer 13 emits a signal cor¬ responding to the oxygen excess O2M (Environment O2) in the flue gases, and the transducer 15 emits a signal corresponding to the NO x content in the flue gases. The transducer signals are low-pass-filtered and A/D-converted, during simultaneous sampling, in a block 19, whose corresponding output signals are supplied to a computer 21 for further processing, as will also be described in more detail further on. The transducer signals are delayed in time both because of the transport time through the suction pipe 17 and because of the inhe- rent dynamics of the respective transducer, which can be described by a time constant. In the present example, the time constant has been selected as the time it takes for the output signal to change from a zero level to a certain level, e.g. about 63% of the final level. The furnace illustrated in Fig. 1 , modified in accordance with an embodiment of the invention, has a control system which is so design¬ ed that the air to the furnace is determined as to its desired value by the steam-pressure regulator of the plant. A low steam pressure calls for a higher main air flow rate to the furnace. The distribution of air between primary and secondary air then takes place in accordance with ratios introduced in the control system. The combustion O2 content measured is then used as desired value for affecting the fuel supply to the furnace. The air control system is supplemented with a PRBS interfe¬ rence function. The desired value of the secondary air flow rate is easily affected by the interfering signal. The new desired value becomes equal to the old desired value *(1 + k * PRBS). Since PRBS is either zero or high, the new desired value means that the secondary air flow rate is caused to increase temporarily when the PRBS signal so determines. In order that the control system now should not counteract the desired secondary air flow rate increase, the main air flow regulator is preferably simultaneously acted on. This mode is referred to as inter¬ ference mode 1. The interfering signal now also affects the main air flow control in such a manner that the actual value of the main air flow regulator is reduced approximately by the amount of air that is taken out via the secondary air flow. The sudden reduction of the artificial actual value of the primary air flow causes the main air flow regulator to increase its effect. In this manner, the primary air flow becomes prac¬ tically unaffected by the interference while the secondary air flow varies. A second mode, interference mode 0, means the same thing as interference mode 1 , however with the difference that the main air flow is unaffected by the interference. This means that the ratio of secondary to primary air flow varies with the interference. The interference used in the project is of the PRBS type. This is an interfering signal of standard type which varies apparently ran¬ domly between two levels. The signal has suitable statistical proper¬ ties by containing a broad range of frequency components. The highest frequency component is so adjusted that it will have time to affect the process. In this application, a minimum period time of the order of a minute, especially 60 seconds, was found suitable. Moreover, there is a set of frequency components that are slower than this fastest period. The frequency spectrum of the signal is a line spectrum with a relatively constant height up to the cut-off frequency. Besides, the PRBS signal has such properties as to be high and low for an equally long time considered over an extended period of time. The PRBS signal can easily be introduced in the configurable control system of the plant as a current signal at suitable intervals. The idea of using controlled interference is to provide higher frequency components in O2 and, hence, in NO x as well. This means that it is possible to make an identification of the relationship between them in a shorter time and with higher accuracy. The employed technique with PRBS interference has proved not to affect the operation of the furnace in other respects. The interfe¬ rence may be connected over long periods of the day, also at night and when the plant is run unmanned. The preferred signal processing which takes place on the basis of the O2M and NO x signals obtained from the transducers 13 and 15 is illustrated in Figs 2 and 3. The signals are filtered by means of hardware as a first step. This type of filter is an LP filter. The signal is thereafter sampled via the A/D converter of the system. Sampling is effected at 1 Hz and low- pass filtration at 0.4 Hz cut-off frequency. The signals NO x and O2M thus received now represent the con¬ tents in ppm and %, respectively. In order to exclude the influence of the dilution, compensation now takes place of NO x to NO x comp, and time differences between the measured signals must now be consi¬ dered. Thus, all the NO x samples are converted into the correspond¬ ing values at 0% oxygen according to the known relationship; NO x comp(t) = NO x (t)/l. - 0 2 M(t-dt)/21.) where t = 1, 2, 3 ... (time in seconds) dt = time compensation. Compensation must be carried out in such a manner that no prompt relationship exists between O2M and NO x comp. Nor must there be any events in time in NO x comp before O2M. Compensation can be exemplified in the following way. Assume that NO x and O2M are stored as 1 -second samples in a respective vector in a file. These figures now contain the dynamics of the transducers. Assuming that the time delay (Transport time + Time constant) =DTO2 of the O2M transducer is, for example, 3 seconds shorter than DTNO x (corresponding assumption for NO x ), then the following rule should be adopted for calculating NO x comp. NO x sample t = 1, 2, 3....s XXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXX O2M sample t = 1, 2, 3, ....s XXXXXXXXXXXXXXXXX NO x comp t= 1, 2, 3, ....s NO x comp (t) = NO x (t)/(l - 0 2 M(t+2)/21) where t = 1, 2, 3....s Now, when O2M, on account of the properties of the trans¬ ducers, precedes NO comp in time, vectors θ2M(t) and NO x comp(t) can be used directly as input signal and output signal, respectively, for subsequent calculation of the transmission function (relationship) and the derivative dNO x comp/dθ2M calculated by step test. Should the NO x transducer be the fastest, i.e. DTNOX be, for example, 2 seconds shorter than DTO2, the order will be reversed in respect of time. Then, the result is instead NO x comp (t) = NO x (t)/(l - O2M(t-2)/21) where t = 3, 4, 5... In this case, a further time compensation must take place before the subsequent calculation so that NO x comp is compensated for in respect of time. The input signal then becomes O2M (t) and the output signal NO x comp(t+2). With these new vectors, identification and step tests can now be performed precisely as above. Should the transducers be exactly equally fast, a time delay by at least one step must all the same be effected, so that no prompt relationship exists between NO x comp and O2M. Hence: NO x comp (t) = NO x (t)/(l - O 2 M(t+l)/21). In this case, no further time compensation is required before the subsequent calculation of the relationship. The thus obtained signals must now be HP-filtered to eliminate the presence of trends and transients. Trends or low-frequency pro- perties of the signals do in fact adversely affect the calculations. The same goes for occurring transients. These do not contain any impor¬ tant information for the subsequent identification. The signals thus processed now have oscillations which principally move about a mean value. The signals can now be used for the recursive identification. In the calculations, an oblivion factor is considered. This means that the values as last sampled are of the greatest importance for the calculations whereas samples previously obtained are weighted to a decreasing extent. Transferred to a time window, the oblivion factor means that a large number of samples, e.g. 2000 samples, form the basis of the calculations. The model thus derived is updated after each new sample. In the preferred case, this means every second. The preferred method used for calculating the relationship between the oxygen content in the flue gases and NO x relies on process identification. The method is the most expediently explained with reference to Fig. 3. The process is perceived as a black box with an input signal and an output signal. The signals are sampled by means of a computer, in which a general model is programmed. The output signal of the model is compared with the output signal of the process. If the difference is zero, the model and the process agree with each other. A deviation immediately results in a correction of the model parameters. It should be noted that the process and the model have the same input signal. The thus obtained model can then be used for determining the dynamics of the process. Step-response tests can be carried out at suitable time intervals and the derivatives can be calculated in this way. All this takes place without interfering with the process. An essential advantage of process identification is that it can be performed as a continuous process. This means that the link between O2 and NO x is calculated on-line and in real time by recursive methods. Changes occurring as a result of variations in the chemistry of the fuel and/ or other changes in respect of combustion technique can thus be detected immediately and suitable measures can be taken in order, for example, to maintain NO x within prescribed limits. For example, the O2 regulator of the control system can be affected on the basis of the derivative for the relationship between O2M and NO x comp in such a manner that the prevailing NO x content of the flue gases is changed in a controlled manner in the desired direction. The preferred model structure used for the process identifica¬ tion is referred to as ARX. The analytical work can be done with the aid of the commercially available software MATLAB. More specific information on the process identification is given in Lennart Ljung, System Identification Toolbox for Use with Mathlab, Users Guide April 6, 1988, the Math- Works, Inc. Fig. 4 illustrates how the present invention can be advan- tageously used for optimising purposes in connection with the injec¬ tion of a chemical, especially urea, with a view to reducing the NO x content of emitted flue gases. The conditions are that the injection of urea can reduce the NO x content, thereby reducing the environmental control charge A for NO x , and that, of course, the cost B for urea injection at the same time increases with an increased urea flow rate U. The total cost of A+B has a minimum corresponding to a certain optimal urea flow rate U 0 pt, which it is thus desirable to be able to determine and adjust towards. This requires a knowledge of the rela¬ tionship between NO x and the urea flow rate U. This relationship can be calculated according to the invention, particularly in the manner described with reference to Figs 1-3, low-pass-filtered and sampled urea flow rate values however replacing after high-pass filtration the O2M values in the relationship calculation itself. Of course, the PRBS interference must also relate to the urea flow rate and not to the air flow rate.

Claims: CLAIMS 1. A method for controlling a combustion process in a combus- tion plant, e.g. a combined power and heating plant, wherein the NO x content of the flue gases generated in the combustion is sensed and the combustion is affected on the basis thereof by controlling at least one parameter in order to maintain the NO x content on the desired level, c h a r a c t e r i s e d by the steps of a) causing interference in said at least one combustion process parameter, preferably the excess of O2 in the flue gases and/ or the flow rate of a chemical, e.g. urea, supplied to the combustion cham¬ ber, and generating a signal representing successive values of the combustion process parameter, b) measuring the NO x content and the O2 content in the flue gases by means of transducers so as to generate an NO x content signal and an O2 content signal, c) performing the NO x and O2 content measurements such that the signals obtained are referable to flue-gas measuring points located at any rate in the immediate vicinity of one another, d) low-pass-filtering the NO x and O2 content signals obtained, e) sampling the low-pass-filtered NO x and O2 content signals to obtain NO x and O2 content values successive in time, f) calculating zero-%-O2-compensated NO x comp content values on the basis of O2 content values obtained, g) using for compensation calculation O2 content values related in time to the corresponding NO x content values, h) high-pass-filtering, preferably softwarewise in a computer, the NO x comp content values obtained and said successive parameter values, so as to eliminate the presence of trends, transients etc., i) calculating, on the basis of the high-pass-filtered values, the relationship between NO x comp and said parameter, and controlling the parameter on the basis of said relationship. 2. A method as claimed in claim 1, c h a r a c t e r i s e d in that the relationship between NO x comp and said parameter is calculated on-line by recursive process identification, wherein the derivative for the relationship is determined by step-response tests. 3. A method as claimed in claim 2, c h a r a c t e r i s e d in that a large number of successive values are used for the recursive process identification, the values last obtained being assigned the highest weight while the values previously obtained are assigned an increasingly lower weight the older they are. 4. A method as claimed in claim 2 or 3, c h a r a c t e r ¬ i s e d in that the model calculation of the process identification is updated after but a few new samples, preferably after each new sample. 5. A method as claimed in any one of claims 2-4, c h a r ¬ a c t e ri s e d in that the time interval between the samples is of the order of seconds. 6. A method as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the sampling of the low-pass- filtered signals is done by A-D conversion. 7. A method as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the interference is caused by means of a so-called PRBS interfering signal. 8. A method as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that, in step g), the relating in time comprises considering the time delay between measuring points and transducers, as well as the time constants of the transducers. 9. A method as claimed in any one of the preceding claims, in which the controlled parameter is the O2 content in the flue gases, c h a r a c t e ri s e d in that said successive parameter values are said O2 content values related in time to the NO x comp content values. 10. A method as claimed in claim 9, in which the combustion process is conducted in a plant using a fluidised bed, preferably a circulating fluidised bed, to which air is supplied in the form of pri¬ mary air and secondary air flows, c h a r a c t e r i s e d in that the interference of the combustion process is provided by affecting the reference value of the secondary air flow rate while simultaneously affecting the main air flow control, so that the primary air flow rate is at least substantially unaffected. 11. A method as claimed in any one of claims 1-9, in which the combustion process parameter is the flow rate of a chemical, especially urea, supplied to the combustion chamber, c h a r a c ¬ t e r i s e d in that the chemical flow rate is controlled, on the basis of the calculated relationship between NO x comp and the sampled chemical flow rate, the cost of the chemical and the cost represented by the NO x environmental control charge, towards a flow rate mini¬ mising the overall cost.

Inventors: BERGDAHL BENGT-GOERAN, BERGDAHL, BENGT-GOERAN
Applicants: BERGDAHL BENGT GOERAN, EUROSIM, BERGDAHL, BENGT-GOERAN
Application year: 1994
Main classification: F23N5
IPC codes: B01D53, F23C10, F23J7, F23J15, F23N5, G05B13